Engine over-temperature warning system

ABSTRACT

An engine over-temperature warning system is especially, although not exclusively, adapted for use in turbine powered helicopters. The system comprises a solid state electronic circuit which is constructed to monitor engine temperatures, and other parameters, so as to provide a warning to the pilot, not only when permissible engine temperatures have been exceeded during starting or in-flight operations, but also when other conditions have occurred which inevitably would lead to engine overheating, so that corrective measures may be taken.

United States Patent 1191 Kelley [4 1 Nov. 27, 1973 ENGINE OVER-TEMPERATURE WARNING SYSTEM [75] Inventor:

[73] Assignee:

Archie P. Kelley, Scottsdale, Ariz.

David L. McPherson, San Diego, Calif.

[22] Filed: Feb. 3, 1972 [21] Appl. No.: 223,195

52] us. or 340/57, 123/41.15, 180/103 581 Field of Search 340/57; 180/103; 123/41.15

[56] References Cited UNITED STATES PATENTS 3,426,322 2/1969 Balo 340/57 3,533,391 10/1970 Lockmuller l23/4l.l5

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3,622,975 ll/l9 7l Vanderberg 340/57 Primary Examiner-John W. Caldwell Assistant Examiner-Glen R. Swann, lIl

AttorneyThomas D. Linton, Jr.

[5 7] ABSTRACT An engine over-temperature warning system is especially, although not exclusively, adapted for use in turbine powered helicopters. The system comprises a solid state electronic circuit which is constructed to monitor engine temperatures, and other parameters, so as to provide a warning to the pilot, not only when permissible engine temperatures have been exceeded during starting or in-flight operations, but also when other conditions have occurred which inevitably would lead to engine overheating, so that corrective measures may be taken.

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TIME (sec) Time (sec) 6- 0011.51- TEMP. @A6E 2O LINEAQ spope IZATE. A P AMP TRlC-IGEQ Low \/o\.TA(=E. DETELTQQ 25 \3 56.1. TIMEIZ 3o Tua-Jzmocoupuz GATE A3 8|As Lb) GATE C) TERMINAL ENGINE OVER-TEMPERATURE WARNING SYSTEM BACKGROUND OF THE INVENTION Accidental over-temperature of turbine engines during starting and in-flight maneuvers of turbine powered helicopters, or other aircraft, is a major cause of premature engine failure and excessive engine wear. Although the pilot normally does have exhaust gas temperature information, and other warning limit instrument information at his disposal, his attention is frequently directed away from his instruments, and especially during emergency power maneuvers, so as to nullify the value of instrument warnings.

Visual and/or audible cockpit alarms have been provided in the prior art for other critical instrument indications. However, in the case of turbine engines, simple over-temperature alarms are not sufficient, since turbine overheating is not a function of temperature alone. Specifically, overheating of a turbine has a time/temperature relationship. That is, a particular temperature may be tolerated for a particular interval of time without creating an overheated condition of the turbine. The permissible time interval shortens as'the temperature rises, until an ultimate temperature threshold is reached which is indicative of turbine over heating, regardless of time duration. Also, present-day temperature sensors, such as thermocouples, and the like, have a time lag associated with their indications which can become critical in the case of turbine engines.

The engine warning system of the present'invention is constructed to provide an audible'or visible alarm whenever a condition occurs with respect to the turbine which inevitably would lead to an overheating condition. The purpose of the alarm system of the invention is to relieve the pilot from thenecessity of constant instrumentation interpretation, and to'predict in advance to the occurrence, the existenceof th'reatened over-temperature conditions, this being achieved by the system monitoring various parameters which indicate a developing over-temperature condition.

Although, as mentioned above,the over-temperature warning system of the invention is particularly applicable to turbine powered helicopter aircraft, it is generally applicable to any piston or turbine engine, as will become evident as the description'proceeds. The warning system to be described is a solid state, analogdigital, miniaturized logic system employing integrated circuit logic elements. The system may behoused in a relatively small casing, as will be described, so that it does not impose space or weight problems.

The system may incorporate a regulated voltage :power supply to permit it to operate withhigh accuracy over a wide range of available input voltages. This is particularly important in 'the case of 'helicopter turbines, because of the wide voltage variations which occur during engine starting under extre'mesinambient conditions, and under conditions of varying battery charge.

One of the parameters monitored by the warning system to be described, for example,-is:the starter voltage. This voltage is monitored and sampledbythesystem'at a critical time of, for example, -l-l Ssecondsafter the start operation. In the event'that thestarter-Voltage is below apredetermined critical value,-as.dete'rminedby engine tests,-during the critical time, an alarmis-triging before actual ignition of the turbine fuel. This isachieved by a' start of heating detector which deactivates the temperature alarm circuitry until heating above a minimum temperature level is achieved.

SUMMARY OF THE INVENTION An over-temperature sensing and alarm system is provided for a turbine engine, or the like, which monitors exhaust gas temperatures, and which includes circuitry for rejecting spurious signals prior to the start of the engine ignition, and which also includes circuitry for discriminating against various different temperature levels as a function of the engine operation specifications. The system of the invention also provides an alarm when the rate of temperature rise exceeds a predetermined value, even though the actual temperature is still below the critical over-temperature threshold. The warning system of the invention also includes circuitry for measuring the electric starter terminal voltage at a given time following initiation of cranking to provide an alarm in the event that this voltage, at that time, is less than a predetermined value so that overtemperature would occur if the starting operation were to be continued.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective representation of a unit which incorporates the engine over-temperature warning system of the invention, and which may be easily and sim- DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT As explained abve, the over-temperature warning system of the invention provides a warning to the pilot of a turbine powered aircraft, for example, that permissible turbine temperature limits will be or have been exceeded. During an attempted start of the engine, the system provides an effective and timely warning regardless of the cause of the overheating. That is, whether the overheating is due to a rich fuel control, high density altitude, low battery condition, dirty compressor, high engine friction, defective starter, or any other malfunction.

'The wamingtime provided by the system of the in- .vention is adequate for the pilot to shut down or cut back on fuel before permissible turbine metal temperatures are exceeded. The advance warning provided by the system of the invention is sufficient to allow for the temperature sensor lag time. Also, when a warning occurs in'flightby the system of the invention, a sufficient time is provided to permit the pilot to reduce power before an in-flightover-temperature condition occurs.

The warning provided by the system is preferably an audible warning, so that the pilot need not shift his attention from the instrument panel.

The system of the invention, as also explained, takes the form of a solid state computer, and it may be housed in a casing, such as shown in FIG. 1, and which is designated 10. The casing has a solid rectangular configuration, and it may have dimensions of the order of as X 3% X 1% inches. In a constructed embodiment, the unit weights about 8 ounces. The system of the constructed embodiment is designed to be operable from 65F to +l60F, and to operate in flight with supply voltages ranging from 12-20 volts DC, and during start with supply voltages ranging from 4 volts to 30 volts DC.

The constructed embodiment includes logic circuitry, as will be described, which operates at 13 seconds i 2 seconds following the closure of the start switch, and is designed to activate the warning unit when the starter terminal voltage is less than 13.0 volts i 1 volt but greater than 5 volts 1- 0.5 volts. The system is also constructed so that at turbine outlet temperatures above 250 C 50C, the warning signal is activated in the event that the rate of turbine outlet temperature rise exceeds 300C per second. At turbine outlet temperatures of 927C t 3%, the system activates the warning when the starter is energized. However, the maximum permissible start temperature is higher than the maximum for in-flight operation. Therefore at turbine outlet temperatures of 749C i 3%, the constructed embodiment of the system activates the warning even when the starter is not energized. The circuitry responds to the higher limit when the start button is depressed. In normal operation the temperature at the turbine outlet drops below the inflight limit at the time the start button is released thus preventing a false alarm.

As mentioned above, the system of the invention monitors the turbine over-temperature during start, as well as during in-flight conditions. The causes of an over-temperature condition during the starting operation are complex and in order to provide an adequate advance warning, the system of the invention evaluates the following parameters: time elapsed following the start initiation; the available starter torque, as represented by the starter terminal voltage during steady state cranking; the turbine outlet temperature; and the turbine outlet temperature rate of rise. These parameters are compared by logic circuitry, such as shown in block form in FIG. 4 and in circuit detail in FIG. 5, with known parameters for a borderline hot start with a given engine, as represented by the curves of FIG. 2. The curve A of FIG. 2 shows the temperature rise with time following ignition during normal starting conditions, whereas the curve B shows the temperature rise with time during borderline hot starting conditions. The borderline hot starting conditions are used for the comparison in the logic circuitry of the monitoring system of the invention.

The system of the invention then makes three different decisions based on the aforesaid comparisons and logic evaluations, which are very similar to those made by the pilot himself during a typical start. Specifically, the alarm is activated if under borderline conditions the cranking rate appears to be below normal, or if the turbine outlet temperature appears to be rising at an excessive rate. Moreover, if at any time the turbine outlet temperature rises above the borderline con dition,as represented by curve B, the alarm is activated. The circuit is also constructed to compensate for the usual thermocouple response of the temperature sensor, as represented by the curve of FIG. 3.

The system to be described switches from the start mode to the in-flight mode"-upon the release of the start button following the'starting operation. The release of the start button automatically switches the system of the invention to inactivate the start logic, and to monitor any over-temperature in the turbine outlet temperature level in excess, for example, of 749C, The aforesaid trigger level in the system during the in-flight mode is pre-set to take care of normal variations between one engine installation and another.

The unit shown in FIG. 1 includes a receptacle 12 which provides connections, for example, for two inputs from the engine thermocouple strip; as well as connections for the DC activating voltage (8+); the starter terminal voltage; a ground connection; as well as an output to an appropriate audible or visual signal alarm unit.

As shown in FIG. 4, the thermocouple strip of the engine, which measures the turbine outlet temperature, is connected to the usual pilot temperature gage 20. The thermocouple strip is connected through the gage 20 to a linear amplifier designated A1 which, in turn, is connected to a slope amplifier A2(a), a start of heating detector A3(a) and an overheat detector A3(b). The starter voltage terminal is connected through a bias gate 22 to the overheat detector A3(b). In the constructed embodiment, for example, the bias gate 22 passes an output to the overheat detector A3(b) when the starter voltage rises above a predetermined minimum.

The starter voltage terminal is also connected to a low voltage detector 24 and to a 13 second timer circuit 25. The output of the voltage detector 24 is connected through an or gate 26 to a silicon controlled rectiver (SCR) gate 28, the output of which is connected to the audible or visual alarm system, The output of the overheat detector A3(b) is directly connected to the aforesaid alarm unit. The slope amplifier A2(a) is also connected to a rate trigger A2(b), the output of which is connected to an and gate 30. The start of heating detector A3(a) is also connected to the and gate 30. The output of the and gate is connected through the or gate 26 to the SCR gate 28.

When the start switch is actuated, the starter voltage is applied to the bias gate 22, to the low voltage detector 24, and to the timer 25. After the interval following the closure of the start switch, which interval is determined by the timer 25, the low voltage detector 24 is enabled to provide an output to SCR 28. In this embodiment, detector 24 provides an output signal if the starter voltage is within the range of 5 to 13 volts. The SCR gate will respond to the output from the low voltage detector 24 to actuate the alarm unit,

The signals from the thermocouple strip of the aircraft are amplified in the linear amplifier A1, and the output from the linear amplifier is applied, as mentioned above, to the slope amplifier A2(a), the start of heating detector A3(a), and the overheat detector A3(b). At turbine outlet temperatures above 250C, for example, the start of heating detector A3( a) will enable the an gate 30. Then, should the rate of turbine outlet temperature rise exceed, for example, 300C per second, the slope amplifier A2(a) actuates the rate trigger circuit A2(b), and a signal is passed through the and gate 30 and through the or gate 26 to activate the SCR gate 28. Therefore, the alarm unit is actuated during the condition of the turbine outlet temperature rise exceeding a predetermined rate, and the temperature of the outlet being in excess of a predetermined threshold.

The output of the linear amplifier A1 is also applied to the overheat detector A3(b) so that an output may be developed at the overheat detector under certain conditions so as to activate the alarm. For example, the bias gate 22 also applies its output to the overheat detector, so that an output will be developed, for example, at a temperature level of 927C, and when the starter is energized. On the other hand, the overheat detector A3(b) will develop an output for in-flight monitoring when the starter is not energized, but when the output from the linear amplifier A1 indicates a turbine outlet temperature in excess of, for example, 759C.

The system of FIG. 4, therefore, performs the desired monitoring functions, and responds to the desired parameters in order to activate the alarm unit in the presence of certain predetermined conditions.

As mentioned above, the system of FIG. 4 is shown in more detail in the circuit diagram of FIG. 5. As shown in FIG. 5, the thermocouple strip of the aircraft is connected to the linear amplifier A1 through a pair of kilo-ohm resistors R11 and R12. The resistor R11 is connected to ground via a 0.22 microfarad capacitor C5, whereas the resistor R12 is connected to an 820 kilo-ohm resistor R13 which, in turn, is connected to the positive terminal of a 5 volt DC regulated voltage source. The linear amplifier A1 is an operational ampli fier, and may be of the type presently designated MCl741G. Its negative input terminal is connected to the resistor R11, and its positive input terminal is connected to the resistor R12. Its number 6 output terminal is connected through a 0.22 microfarad coupling capacitor C7 and through a 2.2 kilo-ohm resistor R15 to the number 2 negative input terminal of the slope amplifier A2(a). The number 6 output terminal of the amplifier Al is connected to the number 2 input terminal through an 820 kilo-ohm resistor R14 which is shunted by a 0.1 microfarad capacitor C6. The number 7 terminal of the amplifier Al is connected to the positive 10 volt terminal of a regulated direct current voltage source. The number 4 terminal is grounded, and the number '1 terminal is connected through a 10 kiloohm balancing potentiometer POTl to the number 5 terminal. The potentiometer PCT] is a null balancing potentiometer, and its armature is grounded.

The output terminal 1 of the amplifier A2(a) is connected through a 10 kilo-ohm resistor R24 to the input terminal 6 of the rate trigger A2(b). The units A2(a) and A2(b) comprise a dual operational amplifier, which may be of the type designated MC 1558G. The positive input terminal number 3 of the amplifier A2( a) is connected through a 2.2 kilo-ohm resistor R16 to the positive 5 volt terminal of 'the regulated DC voltage source. The terminal 4 of the amplifier A2(a) is grounded, and the terminal 8 is connected to the positive terminal of the 10 volt regulated direct voltage source. A 2.2 megohm resistor R17, shunted by a 0.01 microfarad capacitor C18 is connected between the terminals 1 and 2 of the amplifier. The rate trigger amplifier A2(b) has its input terminal connected to a 10 kilo-ohm resistor R25 which, in turn, is connected to the junction of a pair of resistors R and R", the resistors R and R" being connected between the positive terminal of the 10 volt direct current source and ground. The values of the resistors R R are chosen to provide a selected voltage increase rate at which the rate trigger will develop an output. The terminal 8 of the amplifier A2(b) is connected to the positive terminal of the 10 volt direct current source, and the terminal 4 is grounded. The output terminal 7 of the amplifier is connected to a Zener diode CR 2 which may be of the type presently designated IN4729.

The output of the amplifier A1 is also connected to a 10 kilo-ohm resistor R21 which, in turn, is connected to the input terminal 5 of the unit A3(a) which constitutes the start of heating detector. The other input terminal 6 of the unit is connected to a 10 kilo-ohm resistor R20 which, in turn, is connected to the junction of a pair of resistors R18 and R19. The resistor R18 may have a value of 4.3 kilo-ohms, and it is connected to the positive terminal of the 10 volt direct current source. The resistor R19 may have a value of 6.2 kilo-ohms, and it is grounded. The terminal 8 of the unit A3(a) is connected to the positive terminal of the 10 volt direct current source, and the terminal 4 is grounded. The output terminal 7 is connected to a Zener diode CR4 which likewise, may be of the type designated IN4729.

The output of the linear amplifier A1 is also connected through a 10 kilo-ohm resistor R23 to the positive input terminal number 3 of the unit A3(b) which forms the overheat detector. The units A3(a) and A3(b) may be a dual operational amplifier of the type presently designated NC 1558G. The starter terminal voltage is applied to the base of a transistor Q2 through a 10 kilo-ohm resistor R6 and through a l kilo-ohm resistor R7. The junction of the resistors R6 and R7 is connected to ground through a Zener diode CR1. The Zener diode CR1 may be of the type presently designated IN4740A, and it has a 10 volt threshold level.

The transistor Q2 may be of the type presently designated 2N440l. It is an NPN transistor, having its collector directly connected to the positive terminal of the 10 volt DC source, and having its emitter connected through a 5.6 kilo-ohm resistor R8 to the junction of a pair of resistors R9 and R10. The resistor R9 has a value, for example, of 2.64 kilo-ohms and is connected to the 10 volt direct current source. The resistor R10, on the other hand, may have a resistance of 7.32 kiloohms, and it is grounded. The junction of the resistors R9 and R10 is connected through a 10 kilo-ohm resistor R22 to the negative number 2 input terminal of the operational amplifier A3( b) which forms the overheat detector.

The Zener diode CR2 is connected to a 1 kilo-ohm resistor R26 which, in turn, is connected to a diode CR3 and to a grounded 330 ohm resistor R28. The output of the start of heating detector A3(a) is connected to a Zener diode CR4 which, in turn, is connected through a l kilo-ohm resistor R30 to a 330 ohm grounded resistor R31 and to a diode CR5. The Zener diodes CR2 and CR4 may be of the type designated IN4729, and each has a threshold voltage of 3.6 volts. The diodes CR3 and CR5 may be of the type designated IN4003, and they form, in conjunction with their associated circuitry, the and" gate 30 of FIG. 4. The diodes CR3 and CR5 are connected to a 4.7 kilo-ohm resistor R29 which, in turn, is connected to a 3.3 kiloohm resistor R27 to a grounded 10 microfarad capacitor C9. The resistor R27 is connected to the positive terminal of the 10 volt regulated DC source.

The diodes CR3 and CR5 are also connected to a diode CR6 which, together with a further diode CRIO constitute the or gate 26 of FIG. 4. The diodes CR6 and CR1() may be of the type designated 1N4003. The low voltage detector 24 of FIG. 4 is formed by the circuit of an NPN transistor Q3 which may be of the type designated 2N440l. The starter terminal voltage is introduced to a Zener diode CR7 in the low voltage detector. The Zener diode CR7 may be of the type designated IN4742, it has a threshold of the order of 12 volts, so that the low voltage detector 24 will detect starter voltages which are less, for example, than 13 volts. The starter voltage is also introduced to a 390 ohm resistor R32 which is connected to a grounded Zener diode CR8. The Zener diode CR8 may be of the type designated lN4733(a), and it has a threshold voltage of the order of 5 volts, so that the detector 24 provides an output only if the incoming voltage is greater than 5 volts.

The Zener diode CR7 is connected through a 560 ohm resistor R33 to the base of the NPN transistor O3 in the low voltage detector 24. The base is also connected to a 9.1 kilo-ohm grounded resistor R34. The collector of the transistor O3 is connected to the diode CR10 which, as mentioned above, forms part of the or gate 26. The emitter of the transistor Q3 is grounded. in operation, the application of a start voltage greater than about 13 volts to detector 24 causes diode C R7 to break down and drive trans-sistor Q3 into conduction. As a result, the timer 25 output signal is taken to ground through transistor Q3. In the event that the start voltage is less than 5 volts, the threshold of diode CR8 is not exceeded and the timer 25 is not actuated.

The junction of the resistor R32 and Zener diode CR8 is connected to a diode CR9, to a 300 kilo-ohm resistor R36 and to a 330 ohm resistor R37, as well as to the collection of an NPN transistor Q5. The diode CR9 and resistor R36 are connected to the gate electrode of a unijunction transistor Q4 which may be of the type presently designated 2N4853. The gate electrode is also connected to ground via a 22 microfarad capacitor C11. The source electrode of the transistor Q4 is connected to a grounded 47 ohm resistor R38, and the drain electrode is connected to the resistor R37. The source electrode of the transistor Q4 is also connected to a 220 ohm resistor R39 and to a 33 ohm resistor R35.

The resistor R35 is connected to the collector of the transistor Q3, and the resistor R39 is connected to the base of the transistor Q5. The transistor Q3 may be of the type designated 2N440 l and the transistor Q5 may be of the type designated 2N4400. The base of the transistor Q5 is connected to a l kilo-ohm grounded resistor R40, and the emitter is grounded. The circuit of the transistors 04 and Q5 serves as the l3 second timer 26 of FIG. 4. The low voltage detector 24, as formed by the circuit of the transistor 03, develops an output at the end of the 13 seconds established by the circuit of the transistors Q4 and O5, in the event that that the starter terminal voltage, at that time, is less than 12 volts, but greater than 5 volts, these values being selected by way of example only.

The diodes CR6 and CR10 are connected to a grounded 9.1 kilo-ohm resistor R41 and to a grounded 0.22 microfarad capacitor C10 to form the or" gate 26. The diodes are also connected to the base of an NPN transistor Q6. The diodes CR6 and CRlO may be of the type designated IN4003, and the transistor O6 is an NPN transistor, and may be of the type designated 2N440l.

The emitter of the transistor O6 is connected to a grounded 330 ohm resistor R42, and the collector is connected to a 330 ohm resistor R43. The resistor R43 is also connected to a l kilo-ohm resistor R44 and to the base of a PNP transistor Q7. The transistor Q7 may be of the type designated 2N4403. The emitter of the transistor Q7 and the resistor R44 are connected to the starter voltage terminal, as is the anode of a silicon controlled rectifier (SCR) Q8. The SCR 08 may be of the type designated 2N4l5 8. The collector of the transistor 07 is connected to a 33 ohm resistor R45 and to a 0.15 rnicrofarad capacitor C12. The resistor R45 is connected to the gate electrode of the SCR Q8, and the capacitor C12 is connected to the cathode of the SCR and to a diode CR1]. The output of the over heat detector A3(b) is connected to a diode C12, and the two diodes are connected to a grounded 680 kilo-ohm resistor R50 and to a l kilo-ohm resistor R49.

The circuit of the transistors Q6 and Q7, and the SCR 08 form the SCR gate or switching circuit 28, and when an output is derived from the and" gate 30, or from the low voltage detector 24, the transistors Q6 and Q7 are activated to fire the SCR Q8 to establish a voltage across the resistor R50. Likewise, when the over heat detector develops an output, the output voltage is also developed across the resistor R50. Whenever a voltage is developed across the resistor R50, the circuit of the unijunction transistor O9 is activated so as to generate an audio alarm output across the grounded 1.2 kilo-ohm resistor R48 which is connected to the source electrode of the transistor. The transistor Q9 may be a unijunction transistor of the type designated 2N4853.

The circuit of the transistor Q9 is connected to form an audio oscillator, and it includes a 20 kilo-ohm resistor R47 which is connected to the gate electrode of the unijunction transistor Q9 and which is also connected to a 0.] microfarad capacitor C13. A Zener diode CR13 is connected to ground and to the junction of the resistors R47 and R49. The Zener diode CR13 may have a threshold voltage of 9.1 volts, for example, and is of the type designated IN4739. A feedback resistor R46 is connected to the drain electrode and to the junction of the resistors R47 and R49, and the resistor R46 may have a value of 3.3 kilo-ohms.

The invention provides, therefore, an improved overtemperature warning system which is especially adapted for use in turbine powered helicopters, although it has general utility. The system of the invention is advantageous in that it is relatively simple and inexpensive in its construction, and particularly in its ability to compute engine parameters, and provide an advance warning when conditions start to develop which would lead to an overheated state of the engine, unless corrected.

It will also be appreciated that although a particular embodiment of the system has been shown and described, modifications may be made. It is intended in the claims to cover all modifications which fall within the spirit and scope of the invention.

What is claimed is:

1. An over-temperature warning system for use with an internal combustion turbine engine, piston engine, or the like, for monitoring engine outlet temperatures and engine starter voltages to provide an indication when an overheating condition in the engine has occurred, or when an overheating condition inevitably will occur unless corrective measures are taken, said system including: a first input circuit for connection to an electrical temperature transducer which senses the outlet temperature of the engine; a second input circuit for connection to the starter voltage terminal of the engine; an output circuit; and an overheating detector cir cuit coupling said first input circuit and said second input circuit to said output circuit for developing an output signal in said output circuit when the starter of the engine is energized during a condition when said outlet temperature exceeds a predetermined temperature.

2. The warning system defined in claim 1, in which said overheating detector circuit developes an output signal in said output circuit when said outlet temperature exceeds a different predetermined temperature when the starter of the engine is de-energized.

3. The over-temperature warning system defined in claim I which includes slope detection means coupling said first input circuit to said output circuit for developing an output signal in said output circuit when said outlet temperature exceeds a predetermined threshold and is increasing in excess of a predetermined rate.

4. The over-temperature warning system defined in claim 1 which includes a voltage detector and timer circuit coupling said second input circuit to said output circuit for developing a signal in said output circuit a predetermined time after the activation of the starter of the engine if the starter voltage of the engine is less than a first predetermined valve but greater than a second predetermined valve. 

1. An over-temperature warning system for use with an internal combustion turbine engine, piston engine, or the like, for monitoring engine outlet temperatures and engine starter voltages to provide an indication when an overheating condition in the engine has occurred, or when an overheating condition inevitably will occur unless corrective measures are taken, said system including: a first input circuit for connection to an electrical temperature transducer which senses the outlet temperature of the engine; a second input circuit for connection to the starter voltage terminal of the engine; an output circuit; and an overheating detector circuit coupling said first input circuit and said second input circuit to said output circuit for developing an output signal in said output circuit when the starter of the engine is energized during a condition when said outlet temperature exceeds a predetermined temperature.
 2. The warning system defined in claim 1, in which said overheating detector circuit developes an output signal in said output circuit when said outlet temperature exceeds a different predetermined temperature when the starter of the engine is de-energized.
 3. The over-temperature warning system defined in claim 1 which includes slope detection means coupling said first input circuit to said output circuit for developing an output signal in said output circuit when said outlet temperature exceeds a predetermined threshold and is increasing in excess of a predetermined rate.
 4. The over-temperature warning system defined in claim 1 which includes a voltage detector and timer circuit coupling said second input circuit to said output circuit for developing a signal in said output circuit a predetermined time after the activation of the starter of the engine if the starter voltage of the engine is less than a first predetermined valve but greater than a second predetermined valve. 